Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art. Useful background information relating to the chemical vapour deposition of diamond materials may be found in a special issue of the Journal of Physics: Condensed Matter, Vol. 21, No. 36 (2009) which is dedicated to diamond related technology. For example, the review article by R S Balmer et al. gives a comprehensive overview of CVD diamond materials, technology and applications (see “Chemical vapour deposition synthetic diamond: materials, technology and applications” J. Phys.: Condensed Matter, Vol. 21, No. 36 (2009) 364221).
Being in the region where diamond is metastable compared to graphite, synthesis of diamond under CVD conditions is driven by surface kinetics and not bulk thermodynamics. Diamond synthesis by CVD is normally performed using a small fraction of carbon (typically <5%), typically in the form of methane although other carbon containing gases may be utilized, in an excess of molecular hydrogen. If molecular hydrogen is heated to temperatures in excess of 2000 K, there is a significant dissociation to atomic hydrogen. In the presence of a suitable substrate material, diamond can be deposited.
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur. Various methods are available for heating carbon containing gas species and molecular hydrogen in order to generate the reactive carbon containing radicals and atomic hydrogen required for CVD diamond growth including arc-jet, hot filament, DC arc, oxy-acetylene flame, and microwave plasma.
Methods that involve electrodes, such as DC arc plasmas, can have disadvantages due to electrode erosion and incorporation of material into the diamond. Combustion methods avoid the electrode erosion problem but are reliant on relatively expensive feed gases that must be purified to levels consistent with high quality diamond growth. Also the temperature of the flame, even when combusting oxy-acetylene mixes, is insufficient to achieve a substantial fraction of atomic hydrogen in the gas stream and the methods rely on concentrating the flux of gas in a localized area to achieve reasonable growth rates. Perhaps the principal reason why combustion is not widely used for bulk diamond growth is the cost in terms of kWh of energy that can be extracted. Compared to electricity, high purity acetylene and oxygen are an expensive way to generate heat. Hot filament reactors while appearing superficially simple have the disadvantage of being restricted to use at lower gas pressures which are required to ensure relatively effective transport of their limited quantities of atomic hydrogen to a growth surface.
In light of the above, it has been found that microwave plasma is the most effective method for driving CVD diamond deposition in terms of the combination of power efficiency, growth rate, growth area, and purity of product which is obtainable.
A microwave plasma activated CVD diamond synthesis system typically comprises a plasma reactor vessel coupled both to a supply of source gases and to a microwave power source. The plasma reactor vessel is configured to form a resonance cavity supporting a standing microwave. Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions. If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon. Atomic hydrogen can also diffuse from the plasma to the substrate and selectively etch off non-diamond carbon from the substrate such that diamond growth can occur.
A range of possible microwave plasma reactors for diamond film growth via a chemical vapour deposition (CVD) process are known in the art. Such reactors have a variety of different designs. Common features include: a plasma chamber; a substrate holder disposed in the plasma chamber; a microwave generator for forming the plasma; a coupling configuration for feeding microwaves from the microwave generator into the plasma chamber; a gas flow system for feeding process gases into the plasma chamber and removing them therefrom; and a temperature control system for controlling the temperature of a substrate on the substrate holder.
The present inventors consider that when designing a microwave plasma reactor process for diamond film growth, to achieve a successful industrial process requires the assessment of a number of considerations including: chamber and microwave power coupling configuration; gas flow characteristics; and substrate design and temperature control. Certain embodiments of the present invention are primarily concerned with the aspects of substrate design and temperature control.
The most commonly used substrate for CVD diamond growth is silicon. One problem with using silicon as a substrate for CVD diamond growth in a microwave plasma growth process is power absorption by the silicon at high temperatures, leading to thermal runaway and fracture. Another problem is that silicon is readily incorporated into CVD diamond during growth, being particularly visible as the 737 nm Si—V defect. As such, the use of a silicon substrate can detrimentally affect the purity of the CVD diamond product. Yet another problem is that after growth of a CVD diamond wafer on a silicon substrate, recovery of the CVD diamond wafer may require, for example, one of mechanical or acid removal. These additional processing steps increase the time and expense of an industrially implemented process.
In light of the above, it is evident that it would be desirable to find an alternative substrate material which solves these problems.
One possibility for a substrate material is a carbide forming refractory metal such as tungsten, molybdenum, niobium, or alloys thereof. Such substrates have already been proposed in the art. For example, U.S. Pat. No. 5,261,959 suggests a refractory metal substrate material such as molybdenum in the form of a planar circular disk. Alternatively, Whitfield et al. suggest the use of a tungsten substrate (see “Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates”, Diamond and Related Materials, Volume 9, Issues 3-6, April-May 2000, Pages 262-268). Specifically, Whitfield et al. disclose the use of a polycrystalline tungsten disc 6.3 mm thick and 50 mm in diameter and a single crystal tungsten disc 6.3 mm thick and 8 mm in diameter in a 2.45 GHz microwave plasma reactor. The substrates were subjected to preparation steps including polishing to a mirror finish with a 1-3 micrometer diamond abrasive and cleaning via ultrasonic washing and an in situ plasma etch. Substrate temperatures were monitored using optical pyrometry and an embedded thermocouple during CVD diamond growth. Spontaneous delamination of the CVD diamond wafer from the tungsten substrate on cooling after growth is also disclosed to yield a free-standing diamond wafer due to the differences in thermal expansion coefficient between the CVD diamond wafer and the tungsten substrate. Whitfield et al. note that generally in their experiments the substrates were not reused but in the few cases where re-use did occur, substrates were lapped and polished for at least 24 hours to remove the thin carbide layer formed during the previous growth run.
In light of the above, it is evident that carbide forming refractory metals may provide an attractive alternative to silicon substrates. Despite this, the present inventors have experienced a number of problems when using such substrates. These include: non-uniform CVD diamond growth over the substrate; delamination of the CVD diamond wafer from the substrate during CVD diamond growth; and crack initiation and propagation during cooling after growth of the CVD diamond wafer. These problems tend to be exacerbated when larger substrates are used for growing large area polycrystalline diamond discs (e.g. 80 mm diameter or more) or when growing a plurality of single crystal diamonds in a single growth run on a plurality of single crystal diamond substrates adhered to a refractory metal substrate over a relatively large area (e.g. 80 mm diameter or more). This is particularly problematic as there is an on going need to increase the area over which high quality, uniform CVD diamond can be grown. Furthermore, these problems tend to be exacerbated when the substrates are reused in subsequent growth runs. This is particularly problematic as the substrates are expensive and reuse is desirable in an economically competitive industrial process.
It is an aim of certain embodiments of the present invention to at least partially address one or more of these problems. In particular, it is an aim of certain embodiments of the present invention to provide more uniform and/or more consistent CVD diamond products.